Your reaction time when applying the brakes - check your reaction time by taking the simple Reaction Test below.

The following diagram shows the overall stopping distances at a variety of speeds when the road surface is dry. If the road was wet the stopping distance would be twice the distance, on icy roads the stopping distance would be ten times as much.

Thinking Distance - the time it takes you to think about stopping and hit the brakes.
Braking Distance - the time it takes for the car to come to a stop after you have applied the brakes.

Practice makes perfect, so get plenty of practice from a Approved Driving Instructor